In general, a vinyl chloride-based resin latex is obtained by feeding either a vinyl chloride monomer or a monomer mixture composed of a vinyl chloride monomer and one or more monomers copolymerizable therewith to a polymerizer, polymerizing the monomer(s) in an aqueous medium in the presence of a surfactant and a polymerization initiator, causing the polymerization reaction to proceed while keeping the internal temperature of the polymerizer at a given temperature, and terminating the polymerization reaction after the amount of the fed monomer(s) remaining unreacted decreases with increasing conversion into polymer and the internal pressure of the polymerizer has begun to decrease and at the time when the internal pressure of the polymerizer has become a value corresponding to a desired conversion into polymer. Consequently, the vinyl chloride-based resin obtained by drying the vinyl chloride-based resin latex contains the surfactant. Although there are the cases where the surfactant is removed, for example, by washing the vinyl chloride-based resin with water, it has been difficult to sufficiently remove the surfactant. It is known that products produced using resins containing a surfactant in a large amount generally are poor in water resistance and adhesiveness as compared with products produced using resins having a low surfactant content. Furthermore, since unreacted monomers generally remain in vinyl chloride latexes, the unreacted monomers are recovered under heat and reduced-pressure conditions. This operation has had encountered a problem that when the latexes contain a surfactant, considerable frothing occurs to impair the efficiency of this latex treatment.
Some proposals hence have been made on a vinyl chloride-based resin in which the amount of the surfactant remaining therein has been minimized (patent document 1 and non-patent document 1).
In patent document 1, a reactive emulsifying agent having a polymerizable double bond which is capable of chemically combining with the main chain of a resin has been proposed for the purpose of preventing liberation from the resin. In non-patent document 1, soap-free polymerization in which a surfactant is not used at all has been proposed.
However, the reactive emulsifying agent proposed in patent document 1 has had a problem that this emulsifying agent does not wholly combine chemically with a resin and part of the reactive emulsifying agent is still present in the state of having been liberated from the resin. Meanwhile, the technique proposed in non-patent document 1 is mere soap-free polymerization in which although a surfactant is not used at all, the latex is regulated so as to have a solid concentration of 20% or less because polymerization of a vinyl chloride monomer added in an increased amount results in latex coagulation in the course of the polymerization. This technique, when used for actual production, has poor productivity because of the low solid concentration, and has posed a problem because of enhanced scale generation. It is thought that for obtaining a vinyl chloride-based resin latex having a high solid concentration by soap-free polymerization, use may be made of a method in which a vinyl monomer that has a hydrophilic group and is copolymerizable with vinyl chloride is copolymerized with vinyl chloride. Examples of the vinyl monomer having a hydrophilic group and having high copolymerizability with a vinyl chloride monomer are substantially limited to vinyl acetate. Most of the vinyl chloride copolymers which are being produced at present are copolymers of vinyl chloride and vinyl acetate. However, when a monomer mixture composed of vinyl chloride and vinyl acetate is polymerized in the absence of a surfactant, there has been a problem that latex coagulation occurs in the course of the polymerization. Consequently, no vinyl chloride-based resin latex having a high solid concentration and containing no surfactant has been developed.
With respect to vinyl chloride-based copolymers produced from vinyl chloride and a vinyl compound other than vinyl acetate, vinyl chloride-based copolymers produced from vinyl chloride and glycidyl methacrylate are described in patent document 2 and patent document 3.
However, patent document 2 describes a vinyl chloride-based resin containing sodium lauryl sulfate, which is a surfactant. In patent document 3, a vinyl chloride-based polymer containing a polyalkylene glycol, which is a surfactant, is described in the Examples thereof. Namely, each of the vinyl chloride-based resins described in patent document 2 and patent document 3 contains a surfactant. Consequently, neither a vinyl chloride-based resin latex which has a solid concentration of 25% by weight or higher and contains none of those surfactant ingredients nor a process for producing the latex has been developed so far.
Meanwhile, formation of characters or images on a receiving object using a thermal transfer technique has been conducted hitherto. Widely used as the thermal transfer technique are a thermal sublimation type transfer technique and a thermal melting type transfer technique. The thermal sublimation type transfer technique, of these, is a technique in which a sublimable dye is used as a colorant and the dye contained in the sublimable-dye layer disposed on a thermal transfer sheet is transferred to a receiving object, e.g., a thermal transfer image-receiving sheet, by means of a heating device, e.g., a thermal head or a light-emitting laser device, that is controlled so as to generate heat in accordance with image information. In this thermal sublimation type transfer technique, the amount of the dye to be transferred can be regulated with respect to each dot by means of heating conducted in an extremely short period. The image thus formed is exceedingly clear and highly transparent because the colorant used is a dye. The image obtained hence is excellent in terms of half-tone reproducibility and gradation, and exceedingly high-resolution images can be obtained. It is therefore possible to obtain high-quality images comparable to full-color silver-salt photographs. In addition, compared to the ordinary technique of forming silver-salt photographic images, this thermal sublimation type transfer technique has many advantages, for example, that this technique is a dry process, visible images can be obtained directly from digital data, and duplication is easy.
In general, a thermal transfer image-receiving sheet includes at least a support and, formed thereover, a dye-receiving layer (receiving layer) and a heat-insulating layer. As a resin for forming the receiving layer, a vinyl chloride-based resin is being used because this resin is excellent in terms of dyeability and releasability and does not cause abnormal transfer, e.g., fusion bonding, between the thermal transfer sheet and the thermal transfer image-receiving sheet during thermal transfer.
The image transferred to a thermal transfer image-receiving sheet is required to be kept fixed stably over a long period. However, the image transferred to the receiving sheet described above discolors or fades by the action of indoor light or sunlight or as a result of adhesion of greasy matter such as fingerprints. An improvement in image durability hence is desired.
Use of a water-soluble resin or a water-dispersible resin as a resin for receiving-layer formation has been proposed in order to improve image durability. For example, an aqueous dispersion of a polyester resin has been disclosed (patent document 4).
In this case, however, the image has insufficiently improved light resistance although improved in durability in terms of resistance to oily matter.
A proposal hence has been made on addition of a water-dispersible ultraviolet absorber to a water-based coating fluid for receiving-layer formation, and benzotriazole-based and benzophenone-based compounds are shown as examples of the ultraviolet absorber. However, such organic compounds having ultraviolet-absorbing ability have a relatively low molecular weight and hence are unstable, and there have been the cases where the receiving sheet in the unprinted state yellows when stored over a long period. A proposal has hence been made on addition of a water-soluble polymeric ultraviolet absorber (patent document 5).
However, there has been a problem that when such a high-molecular ultraviolet absorber is added in a large amount in order to sufficiently improve light resistance, the result is a decrease in water resistance.